This is a competitive renewal application for a project (NS35120-04) on development of new single photon emission computed tomography (SPECT) imaging agents to evaluate serotonergic neurotransmission of the central nervous system (CNS). Serotonin (5-HT) is an essential neurotransmitter in the brain, which controls various important behaviors, including sleep awake cycle, mood, temperature, appetite, etc. In addition, commonly prescribed antidepressants, selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, sertraline and paroxetine, modulate 5-HT neurotransmission by blocking the serotonin transporters (SERT). Imaging of SERT in humans would provide a useful tool to understand how alterations of this system are related to depressive illness and other psychiatric disorders and therefore, potentially can benefit millions of patients who are being treated with SSRIs. Development of selective tracers for positron emission tomography (PET) and single photon emission tomography (SPECT) have made it possible to study in vivo neuroreceptors or specific transporter sites non-invasively in the human brain. The major objective of this project is to develop specific radioactive iodinated (I-123) tracers for in vivo SPECT imaging of SERT in the brain. A novel radioiodinated agent, 5-iodo-2-(2-(dimethylamino)methyl)phenyl)thio)-benzly alcohol (IDAM), was recently developed in our laboratory. IDAM displayed a high binding affinity to SERT (Ki = 0.097 nM). The binding affinity to the other two monoamine transporters is much lower (Ki = > 10,000 and 234 nM for dopamine and norepinephrine transporters, respectively). This novel tracer, who demonstrated a high affinity, excellent selectivity and good brain penetration, has excellent characteristics for SPECT imaging of SERT in the brain. Synthesis and structure-activity relationship studies of a series of IDAM derivatives are proposed. Selected compounds will be labeled and tested for in vivo biodistribution in rats and SPECT imaging study in non-human primates. In addition, behavioral study (forced swim test in rats) specific for antidepressive effect and microdialysis study (at the striatum of rat brain) for measuring the elevation of endogenous 5-HT levels will be performed to confirm the pharmacological mechanisms of the new SERT inhibitors. The novel imaging agents may lead to simple and useful routine diagnostic procedures for various mental health diseases involving SERT in the brain.